Arrangement for recovering a clock from a modulated optical input signal

ABSTRACT

A laser arrangement for recovering a clock from a modulated optical input signal (S o ) has, between a resonator mirror (21) for launching the input signal and a resonator mirror (42) for coupling out the recovered clock (T), a bidirectional optical transmission channel (3) having at least two nonlinear optical amplifiers (2, 4) and a demultiplexer (6) between the amplifiers. The arrangement is operated as a mode-locked laser and is suitable for optical repeaters.

Modulated optical input signal clocked by this clock on an optical inputcarrier wavelength are disclosed, for example, in D. M. Patrick, R. J.Manning: 20 Gbit/s all-optical clock recovering using semiconductornonlinearity, Electr. Lett. Vol. 30 (1994), pages 151 to 152; A. D.Ellis, K. Smith, D. M. Patrick: Alloptical clock recovery at bit ratesup to 40 Gbit/s, Electr. Lett. Vol. 29 (1993), pages 1323 to 1324 or P.E. Barnsley, E. J. Wickes, E. G. Wickens, D. M. Spirit: All-opticalclock recovery from 5 Gb/s RZ Data using a self-pulsating 1.56 μm Laserdiode, IEEE Phot. Tech. Lett., Vol. 3 (1991) pages 942 to 945, the clockbeing recovered in these arrangements by mode locking of a laser to themodulated input signal. A nonlinear optical element is always requiredfor the purpose of mode locking in an optical resonator. Use is made, asnonlinear optical elements, of optical amplifiers (see Electr. Lett.Vol. 30 (1994), pages 151 to 152), optical fibers (see Electr. Lett.Vol. 29 (1993), pages 1323 to 1324), saturable absorbers (see IEEE Phot.Tech. Lett., Vol. 3 (1991), pages 942 to 945) and sections ofmultisection DFB lasers (see U. Feiste, D. J. As, A. Erhardt: 18 GHzall-optical frequency locking and clock recovery using a self-pulsatingtwo-section DFB laser, IEEE Phot. Tech. Lett. Vol. 6 (1994), pages 106to 108 or R. J. Manning, D. A. O. Davies, D. Cotter, J. K. Lucek:Enhanced recovery rates in semiconductor laser amplifiers using opticalpumping, Electr. Lett. Vol. 30 (1994), pages 787 to 788).

The data rate of saturable absorbers has so far been limited toapproximately 5 Gb/s (see IEEE Phot. Tech. Lett., Vol. 3 (1991), pages942 to 945).

Data rates of 40 Gb/s have already been achieved using optical fibers.However, with fiber lengths of approximately 10 km, they are difficultto integrate.

20 Gb/s (see Electr. Lett. Vol. 30 (1994), pages 151 to 152) have beenachieved using optical amplifiers in the resonator, and a figure of 18GHz (see IEEE Phot. Tech. Lett. Vol. 6 (1994), pages 106 to 108) hasbeen achieved using multisection DFB lasers.

An advantage of an arrangement having an optical amplifier in theresonator as compared with multisection DFB lasers is that the nonlinearelement is more effectively separated from the residual resonator withits amplifying medium, and can therefore be specifically influenced.Thus, a proposal has already been made as to how the clock can bebrought up to 100 Gb/s using an optical amplifier in the resonator,specifically by intensive pumping of the nonlinearly operating opticalamplifier using a continuously operating laser (see Electr. Lett. Vol.30 (1994), pages 787 to 788).

SUMMARY OF THE INVENTION

The invention, whose generic concept specified in the preamble isdisclosed in Electr. Lett. Vol. 30 (1994), pages 151 to 152, or elseElectr. Lett. Vol. 30 (1994), pages 787 to 788, has the advantage thatthe laser arrangement can be integrated in a module and that it isrendered possible to recover the clock from the modulated input signalin optical networks with high data rates of 10 Gb/s and more.

The arrangement according to the invention, which can be designated asan optical clock, corresponds to a mode-locked laser which has anoptical resonator with a wavelength-selective optical transmissionchannel section and at least one optical amplifier at each end of thischannel section, the position of the wavelength-selective opticaltransmission channel section in the resonator being essential to theinvention.

The optical transmission channel section can advantageously be realizedby a bidirectional optical wavelength demultiplexer which acts as ademultiplexer when operated in one direction and acts as a multiplexerwhen operated in the opposite direction.

The arrangement according to the invention can advantageously be used toregenerate the optical input signal in an optical repeater.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention which are believed to be novel,are set forth with particularity in the appended claims. The invention,together with further objects and advantages, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings, in the several Figures of which like referencenumerals identify like elements, and in which:

FIG. 1 shows a first exemplary embodiment of an arrangement according tothe invention,

FIG. 2 shows a second exemplary embodiment of an arrangement accordingto the invention, and

FIG. 3 shows a third exemplary embodiment of an arrangement according tothe invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The laser arrangement according to the invention and represented by wayof example in FIGS. 1 to 3 recovers the clock T from a modulated opticalinput signal S_(o) clocked by this clock T on an input carrierwavelength λ₂, and has an optical resonator 1 composed of abidirectional optical transmission channel 3 of predetermined opticallength L and extending between two resonantor mirrors 21 and 42.

The transmission channel 3 has, between the two resonator mirrors 21 and42, at least two nonlinear optical amplifiers, for example the twoamplifiers 2, 4 according to FIG. 1 and, between these amplifiers 2, 4,a wavelength-selective transmission channel section 31.

The two amplifiers 2, 4 are arranged sequentially in a transmissiondirection r of the transmission channel 3 pointing from one resonatormirror, for example the resonator mirror 21, to the other resonatormirror, to the resonator mirror 42 in the case of the example.

The transmission channel section 31 connects the two amplifiers 2, 4optically and serves to transmit an optical signal on a predeterminedoptical carrier wavelength between these two amplifiers 2, 4, forexample the signal S₁ ' on the predetermined carrier wavelength λ₁ inFIG. 1.

On the basis of the transmission direction r specified above--theopposite direction could also be adopted--the input signal S_(o) on theinput carrier wavelength λ₂ is launched into the transmission channel 3by the resonator mirror 21. The other resonator mirror 42 serves tocouple out an optical signal S₁ on the predetermined carrier wavelengthwhich originates from the second and last amplifier 4 in the adoptedtransmission direction r.

Together with the first amplifier 2, the last amplifier 4 and thewavelength-selective transmission channel section 31, the resonator 1forms a mode-locked laser if the first amplifier 2 operates in thenonlinear region with respect to the launched input signal S_(o). Forthis purpose, the light power, for example, of the input signal S_(o) isselected to be so high that the first amplifier 2 operates in thenonlinear region, that is to say the optical signal power is higher thanthe saturation power of the first amplifier 2. A gain then occurs in theresonator 1 which is modulated for all optical wavelengths with thesignal rate of the input signal S_(o), and the longitudinal laser modesof the laser arrangement are coupled to one another, with the resultthat pulses with the clock rate of the input signal S_(o) occur. Thepulse width of the mode-locked laser is a function of the modulation ofthe input signal S_(o), of the bit sequence and of the bandwidth of thewavelength-selective transmission channel section 31.

The optical signal S₁ originating from the last amplifier 42 and coupledout by the other resonator mirror 42 has pulses I which occur inherentlyperiodically in a clock cycle t of the clock T and define the coveredclock T.

In order to adapt the propagation time of the pulses through theresonator 1 to the clock cycle t of the clock T of the input signalS_(o) in such a way that it is an integral multiple of this clock cyclet, the optical length L of the transmission channel 3 of the resonator 1can be adapted, for example, by trimming the temperature of an opticalamplifier 2 and/or 4.

There is only one last optical amplifier 4 in addition to the firstamplifier 2 in the example according to FIG. 1.

However, in the example according to FIG. 1, the transmission channel 3has an additional wavelength-selective transmission channel section 32which is arranged between the first amplifier 2 and the other resonatormirror 42 and connects the first amplifier 2 optically directly to theother resonator mirror 42. This additional transmission channel section32 is designed in such a way that the other resonator mirror 42 is fedan optical signal S_(o) ' on the intput carrier wavelength, which signaloriginates from the first amplifier 2 λ₂, can be coupled out of theother resonator mirror 42 and is inherently clocked by the clock T andis modulated in accordance with the input signal S_(o) such that itcorresponds to the launch signal S_(o).

In the example according to FIG. 1, the arrangement could also be setup, for example, such that the transmission channel section 31 connectedto the last amplifier 4 transmits only the input carrier wavelength λ₂,and the additional transmission channel section 32 connected to theother resonator mirror 42 transmits only the carrier wavelength λ₁. Inthis case, which is not represented in FIG. 1, it would be possible tocouple out of the resonator mirror 42, on the one hand, an opticalsignal S₂, originating from the first amplifier 2 and transmitted viathe transmission channel section 31, on the input carrier wavelength λ₂and, on the other hand, an optical signal S₁, originating from the firstamplifier 2 and transmitted via the additional transmission channelsection 32, on the carrier wavelength λ₁. The signal S₂ on the inputcarrier wavelength λ₂ would have pulses I which occur inherentlyperiodically in the clock cycle t of the clock T and define therecovered clock T, and the signal S₁ on the carrier wavelength λ₁ wouldbe a signal clocked inherently by the clock T and modulated inaccordance with the input signal S_(o). This signal S₁ would correspondto the signal S_(o) ' or the input signal S_(o) which is converted fromthe input carrier wavelength λ₂ to the carrier wavelength λ₁.

A wavelength conversion is generally possible in the case where signalson different carrier wavelengths are coupled out of the other resonatormirror 42.

A wavelength conversion of the input signal S_(o) can also be performedby modulating in accordance with the input signal S_(o) an opticalsignal on a carrier wavelength which originates from a last amplifier 4,is coupled out of the resonator mirror 42 and has pulses which occurinherently periodically in the clock cycle of the clock and define therecovered clock T, for example the signal S₁ on the carrier wavelengthλ₁ according to FIG. 1. This modulated signal would correspond to theinput signal S_(o) which has been converted to this carrier wavelengthfrom the input carrier wavelength λ₂.

The exemplary embodiment according to FIG. 2 differs from the exemplaryembodiment according to FIG. 1 essentially in that, between the tworesonator mirrors 21 and 42, the transmission channel 3 has, in additionto the first nonlinear optical amplifier 2, two last nonlinear opticalamplifiers 4, which are arranged downstream of the first amplifier 2 inthe transmission direction r, as well as a wavelength-selectivetransmission channel section 31 which is arranged between the firstamplifier 2 and the two last amplifiers 4, connects the first amplifier2 optically to each last amplifier 4 and is designed in such a way thatone optical signal S₁, S₂ each is transmitted between the firstamplifier 2 and each last amplifier on a predetermined carrierwavelength λ₁ and λ₂, respectively, assigned to this last amplifier 4,it being the case that the other resonator mirror 42 serves to coupleout of the transmission channel 3 on the predetermined carrierwavelength λ₁ and λ₂, respectively, assigned to this last amplifier 4the optical signal S₁, S₂ which originates from each last amplifier 4and which has pulses I which occur inherently periodically in the clockcycle t of the clock T and define the recovered clock T.

Specifically, in FIG. 2 the carrier wavelength λ₁ is assigned to theupper last amplifier 4, and the input carrier wavelength λ₂ is assignedto the lower last amplifier 4. In this specific case, the recoveredclock T can be connected between the carrier wavelengths λ₁ and λ₂.

Two or more such last optical amplifiers 4 generally provide thepossibility of switching the carrier wavelength of the recovered clock Ton a predetermined raster.

Wavelength conversion would also be possible in the case of the exampleaccording to FIG. 2 if the output signal S₁ were modulated in accordancewith the input signal S_(o).

The exemplary embodiment according to FIG. 3 differs from the exemplaryembodiment according to FIG. 2 essentially in that the lower lastamplifier 4 is assigned not the input carrier wavelength λ₂, but acarrier wavelength λ₃ which differs both therefrom and from the carrierwavelength λ₁ assigned to the upper last amplifier 4, and in that, as inthe example according to FIG. 1, the transmission channel 3 has anadditional wavelength-selective transmission channel section 32 which isarranged between the first amplifier 2 and the other resonator mirror 42and connects the first amplifier 2 optically directly to the otherresonator mirror 42. This additional transmission channel section 32 isdesigned in such a way that the other resonator mirror 42 is fed on theinput carrier wavelength λ₂ an optical signal S_(o) ' which originatesfrom the first amplifier 2, can be coupled out of the other resonatormirror 42 and is inherently clocked by the clock T and is modulated inaccordance with the input signal S_(o) such that it corresponds to thelaunch signal S_(o).

It is possible to couple out of the resonator mirror 42 on the carrierwavelength λ₃ assigned to the lower last amplifier 4 an optical signalS₃ which originates from this lower last amplifier 4 and has impulses Iwhich occur inherently in the clock cycle t of the clock T and definethe recovered clock T. In this specific case, the recovered clock T canbe connected between the carrier wavelength λ₁ and λ₃. Here, as well,wavelength conversion is possible if at least one of the two signals S₁and S₃ is modulated in accordance with the input signal S_(o).

In order to be able to drive the clock rate of the clock T recoveredfrom the optical clock as high as possible, the nonlinearly operatingfirst optical amplifier 2 must be illuminated by an intensive undampedoptical wave W of a further optical wavelength λ₅ which differs bothfrom the input carrier wavelength λ₂ and from the carrier wavelengthsλ₁, λ₃ which differ from this wavelength λ₂ (see Electr. Lett. Vol. 30(1994) pages 787 to 788). This further optical wavelength λ₅ either canbe launched into the resonator 1 via an existing additional opticaltransmission channel section 32, and then radiates against the launchedinput signal S_(o), or is mixed with the input signal S_(o) and thenseparated from the launched signal and emitted at another point.

In the examples according to FIG. 1 and FIG. 3, the set-up is such thatthe undamped wave W is launched into the first amplifier 2 by oneresonator mirror 21. Coupled out of the first amplifier 2 is an opticalwave W' which corresponds to the launched wave W, has the same opticalwavelength λ₅ as the latter, is fed to the other resonator mirror 42 viathe additional transmission channel section 32 and can be coupled out ofthis resonator mirror 42. The additional transmission channel section 32according to FIG. 1 and FIG. 3 is designed for this reason in such a waythat, in addition to the input carrier wavelength λ₂, it also transmitsthe wavelength λ₅ of the optical wave W.

In the example according to FIG. 2, by contrast, the undamped wave W islaunched into the first amplifier 2 by the other resonator mirror 42 viathe additional transmission channel section 32.

The transmission channel section 31 and/or additional transmissionchannel section 32 are/is preferably realized by a bidirectional opticalwavelength demultiplexer 6, an integratable demultiplexer beingpreferred.

A method which can be used to integrate a demultiplexer with activeoptoelectronic elements is specified in the published European PatentApplication 0 497 358.

One resonator mirror 21 of the resonator 1 is preferably a partiallyreflecting amplifier input of the first optical amplifier 2. The otherresonator mirror 42 of the resonator 1 is preferably a partiallyreflecting mirror which comprises a partially reflecting opticalamplifier output of one or more last amplifiers 4.

One amplifier output 22 of the first optical amplifier 2 and theamplifier input 41 of each last optical amplifier 4, to which thetransmission channel section 31 is optically coupled, are preferablyoptically coated.

The invention is not limited to the particular details of the apparatusdepicted and other modifications and applications are contemplated.Certain other changes may be made in the above described apparatuswithout departing from the true spirit and scope of the invention hereininvolved. It is intended, therefore, that the subject matter in theabove depiction shall be interpreted as illustrative and not in alimiting sense.

What is claimed is:
 1. A laser arrangement for recovering a clock from amodulated optical input signal, the input signal being clocked by saidclock on an optical input carrier wavelength, comprising:an opticalresonator having a bidirectional optical transmission channel that has apredetermined optical length and that extends between two resonatormirrors; at least first and second nonlinear optical amplifiers arrangedin the transmission channel between the resonator mirrors; an inputsignal that is coupled into the transmission channel by a first mirrorof the two resonator mirrors, the input signal being amplified opticallyin the first nonlinear optical amplifier; an optical signalrepresentative of a recovered clock, the optical signal being coupledout of the transmission channel by a second mirror of the two resonatormirrors; the at least first and second nonlinear optical amplifiersarranged sequentially in a transmission direction of the transmissionchannel; a wavelength-selective transmission channel section locatedbetween the at least first and second nonlinear optical amplifiers and,connecting the amplifiers; an optical signal on a predetermined opticalcarrier wavelength being transmitted in the channel section; the firstresonator mirror inserting the input signal into the transmissionchannel on the input carrier wavelength; the first amplifier in thetransmission direction operating in a nonlinear region with respect tothe input signal; and the second resonator mirror coupling out of thetransmission channel the optical signal coupling out of the transmissionchannel the optical signal on the carrier wavelength that is determinedby the transmission channel section, the optical signal originating froma last amplifier of the at least first and second amplifiers in thetransmission direction and having pulses which occur periodically in aclock cycle of the clock and define a recovered clock.
 2. Thearrangement as claimed in claim 1, wherein, between the two resonatormirrors, the transmission channel has a first nonlinear opticalamplifier and at least two last nonlinear optical amplifiers, which arearranged downstream of the first amplifier in the transmissiondirection, as well as a wavelength-selection transmission channelsection which is arranged between the first amplifier and the at leasttwo last amplifiers, the channel section connecting the first amplifieroptically to each last amplifier and being structured such that arespective optical signal is transmitted between the first amplifier andeach last amplifier on a predetermined carrier wavelength assigned toeach last amplifier;the second resonator mirror coupling out of thetransmission channel on the predetermined carrier wavelength assigned toeach last amplifier a respective further optical signal which originatesfrom each last amplifier and which has pulses which occur inherentlyperiodically in a clock cycle of the clock and define the recoveredclock.
 3. The arrangement as claimed in claim 2, wherein predeterminedcarrier wavelengths assigned to different last amplifiers differ fromone another.
 4. The arrangement as claimed in claim 1, wherein eachpredetermined carrier wavelength respectively assigned to a lastamplifier differs from the input carrier wavelength.
 5. The arrangementas claimed in claim 1, wherein a predetermined carrier wavelengthassigned to a last amplifier is equal to the input carrier wavelength.6. The arrangement as claimed in claim 1, wherein the optical signal,coupled out of the second resonator mirror and originating from a lastamplifier, is modulated on the predetermined carrier wavelength assignedto this last amplifier.
 7. The arrangement as claimed in claim 1,wherein the transmission channel has an additional wavelength-selectivetransmission channel section which is arranged between the firstamplifier and the second resonator mirror, which connects the firstamplifier optically directly to the second resonator mirror and which isstructured such that there are transmitted between the first amplifierand the second resonator mirror at least two optical signals which havemutually differing predetermined optical wavelengths and can be coupledout of the second resonator mirror, the at least two optical signalsbeing spatially separated from one another.
 8. The arrangement asclaimed in claim 7, wherein, via the additional transmission channelsection, the second resonator mirror is fed a respective further opticalsignal on the input carrier wavelength, the respective further opticalsignal originating from the first amplifier, being coupled out of thesecond resonator mirror and being clocked by the clock and modulated inaccordance with the input signal.
 9. The arrangement as claimed in claim1, wherein, in addition to the input signal an undamped optical wave issupplied to the first amplifier, and wherein the undamped optical wavehas a further optical wavelength, which differs both from the inputcarrier wavelength and from the predetermined carrier wavelengths. 10.The arrangement as claimed in claim 9, wherein the undamped wave iscoupled into the first amplifier by the first resonator mirror.
 11. Thearrangement as claimed in claim 9, wherein a further undamped opticalsignal is coupled out of the first amplifier, and wherein the opticalfurther undamped signal, which relates to the undamped optical signalsupplied to the first amplifier, and which has the same opticalwavelength as the optical wavelength of the undamped signal supplied tothe first amplifier, is fed to the second resonator mirror via theadditional transmission channel section and coupled out of the secondresonator mirror.
 12. The arrangement as claimed in claim 9, wherein theundamped optical signal is supplied to the first amplifier by the secondresonator mirror via the additional transmission channel section. 13.The arrangement as claimed in claim 1, wherein the transmission channelsection is realized by a bidirectional optical wavelength demultiplexer.